After four years of grant support, we have enrolled three cohorts of twin families consisting of 11 year old twins and their mother and father. To date we have had 336 family visits to the study. The attrition rate from the first visit to the second visit has been 3.7%. The overall goals of this design are to analyze the genetic and environmental contributions of juvenile hemodynamic determinants of blood pressure to adult cardiovascular (CV) risk. These goals will be achieved by a laboratory study of hemodynamic measures in a longitudinal study of 1,200 juvenile twins age 9.5 to 17 years and their parents. The specific hypotheses represent a progressive approach to hypothesis testing beginning with 1) an epidemiologic analysis which does not exploit familial resemblance; 2) a more refined univariate analysis of genetic and environmental effects exploiting the unique aspects of the twin-parent design; 3) an analysis of the developmental change in the genetic and environmental effects using the power of longitudinal designing; and 4) a multivariate developmental analysis of the interrelationships among the cardiovascular variables. The analysis will address the following general questions: 1) What are the relationships between hemodynamic variables and family history of CV disease to the level of blood pressure in children. 2) What are the contributions of genes and environment to family resemblance. 3) Do genetic and environmental effects upon blood pressure change during development. 4) How do genes and environment contribute to the interrelationships between blood pressure and its hemodynamic determinants.